On your iOS device download the OpenVPN Connect app
You can use this riseup.ovpn configuration file with OpenVPN Connect.
The tricky part is that you need to get the riseup.ovpn file to your device. The Connect app lists a few ways: the simplest is to email it to yourself, but be cautious about doing this with insecure email. Transferring via Dropbox, etc also works. In the share sheet, open the file with OpenVPN Connect.
Tap the green + to add the profile to OpenVPN Connect. Start the connection and enter your “Riseup Red” username and password. Alternately, you can configure a separate VPN password.